This invention is in the technical field of controlling the light emission from a flash lamp internally contained in or externally attached to a digital cameral adapted for use in underwater photography.
When a camera (such as a digital camera) is used for underwater photography, it is usually enclosed inside a commercially available transparent colorless watertight housing. A transparent colorless container is used because the flash light emitted from a stroboscopic lamp internally provided to the camera will have to pass through the material of the housing to reach the target object to be illuminated. A stroboscopic lamp (xe2x80x9cflash lampxe2x80x9d) is more frequently used by an underwater photographer because the underwater environment is generally darker than above the water. If an internally provided flash lamp (xe2x80x9cinner lampxe2x80x9d) is used, however, the emitted flash light is reflected by dust particles and microorganisms floating in water because the light emitting element of the inner lamp is positioned close to the lens of the camera 1, producing white spots in the image and thereby giving rise to the so-called marine snow phenomenon. If the existing particles are very fine, the picture as a whole may become whiter, adversely affecting the contrast of the picture.
An externally attached flash lamp (xe2x80x9couter lampxe2x80x9d) is used in view of these problems. Since the light emitting element of the outer lamp can be farther removed from the lens of the camera, reflected light from particles near the lens can be reduced and the effect of the marine snow phenomenon becomes less severe, allowing it to be possible to obtain clearer pictures.
The explanations given above are generally on the assumption that the target objects are at distances in the range of 0.5 m-1.5 m.
FIG. 1 shows a prior art method of control of a digital camera 1 provided with a stroboscopic light emitting element (or the inner lamp) 2 and a lens 3 and enclosed inside a watertight housing 4 to which an outer lamp 9 is attached. Numeral 7 indicates its outer frame. The outer lamp 9 must be synchronized with the so-called X-contact of the camera 1 (which is a switch on the part of the camera 1 adapted to close when the shutter of the camera 1 is opened) but commercially available camera housings are not equipped with any synchronization terminal to be connected to an outer lamp because it is troublesome to waterproof the portion of the housing at such a terminal and also because some cameras are not provided with an X-contact terminal.
A simple method of synchronizing the light emission from the outer lamp 9 without a synchronization terminal is to provide the outer lamp 9 with a sensor 10 for detecting the light emitted from the inner lamp 2 such that the emission of light from the outer lamp 9 will be synchronized. Numeral 8 indicates an optical fiber. The emission of light from the outer lamp 9 can be synchronized even without the optical fiber 8 as long as the light from the inner lamp 2 can reach the sensor 10. If the distance between the light emitting part of the inner lamp 2 and the outer lamp 9 is large, however, the optical fiber 8 serves to dependably transmit to the sensor 10 the light from the inner lamp 2.
In order to reduce the effect of the aforementioned marine snow phenomenon, a black light-screening plate 6, or a plate with a black tape pasted thereon, is provided in front of a diffusing plate 5 such that the light from the inner lamp 2 will not reach the target object for the reason given above. According to the example shown in FIG. 1, the light emitted from the inner lamp 2 is passed through the optical fiber 8 to be detected by the outer lamp 9. This method is disadvantageous, however, in that the optical fiber must be provided and it is costly to have it installed.
The user of a camera as shown in FIG. 1 will discover that a significant amount of light emitted from the inner lamp 2 is leaking through the portions around the light-screening plate. Even if the light-screening plate is made wider, light continues to leak around the light-screening plate because the watertight housing is transparent and colorless. It is also because the walls of the light-emitting part of the camera and the watertight housing are separated. This gives rise to the aforementioned marine snow phenomenon because this leaked light passes very close to the lens of the camera. The marine snow phenomenon occurs also if there are many light-reflecting particles, whether the distance to the target object to be photographed is large or small. The light-screening plate is not very effective, although it is better than not using it at all.
Another problem of the prior art technology is the so-called flare phenomenon with a part of the light from the inner lamp reaching the lens of the camera by being reflected inside the housing or diffused near by because the housing is colorless and transparent. As a result, the user may fail to obtain a picture with a good contrast.
In the case of a digital camera for underwater photography, the camera itself is made watertight such that a housing as shown in FIG. 1 is not required. Since the inner lamp must be caused to emit light, however, there remains the same problem of the marine snow phenomenon unless the inner lamp and the outer lamp are connected by a synchronization cable for synchronizing their operations.
It is therefore a general object of this invention, in view of the status of the prior art technology described above, to provide a device for and method of controlling the emission of light from a flash lamp externally attached to a digital camera (xe2x80x9cthe outer lampxe2x80x9d) for preventing as much as possible the occurrence of marine snow and flare phenomena caused by the light from an internally provided lamp (xe2x80x9cthe inner lampxe2x80x9d) of the camera such that pictures of a higher quality than was possible before can be obtained.
It is a more specific object of this invention to provide such a device and a method characterized as using infrared light (inclusive of near-infrared light) to which the image-taking element of the camera is insensitive.
It is another more specific object of this invention to provide such a device and a method characterized as reducing the quantity of light coming from the light emitting part of the internally provided lamp of the camera or its vicinity so as not to be photographed, or as using a light guide to lead the flash light away so as not to be incident on the target objects to be photographed.
In view of the objects of the invention as described above, one of the methods of the invention may be described as comprising the steps of preparing a filter which passes infrared light but prevents visible light from passing through, providing the light emitting part of the inner lamp of a camera (such as a digital camera for underwater and/or above-water photography) with this filter, causing infrared light emitted through this filter to be received by the light receiving part of the outer flash lamp and thereby causing the light receiving part to control emission of light from the outer flash lamp. The filter may be pasted on the front surface of the light emitting part of the inner lamp or on the inner or outer surface of a watertight housing in which the camera may be enclosed when it is used for underwater photography. A control device to be used correspondingly may thus be characterized as comprising such a filter which passes infrared light but prevents visible light from passing through, being provided to the light emitting part of the inner lamp and a light receiving element which is in the light receiving part of the light receiving part and serves to control emission of light from the outer lamp in response to infrared light received through the filter.
In summary, this aspect of the invention is characterized as using the infrared (inclusive of near infrared) portion of the flash light from the inner lamp to generate a signal for activating the outer flash lamp. The outer lamp usually comprises a xenon discharge tube but xenon discharge tubes are well known for emitting infrared light equally strongly as or more strongly than visible light. According to this aspect of the invention, the visible portion of the emitted light from the inner light is filtered off and the abundantly present infrared portion is used to activate the outer flash lamp. For this purpose, the light receiving part of the outer lamp must include an element which has sensitivity to infrared light, or the capability of generating an electrical signal for activating the outer lamp in response to the infrared light received through the filter. Since silicon phototransistors and silicon photodiodes of known kinds have sufficient sensitivity to infrared light, they may directly be used for the purpose of this invention.
In another aspect of the invention, use is made of a light intensity-reducing filter, instead of a filter for preventing visible light from passing through while allowing infrared (and near infrared) light to pass through, either pasted on the light emitting part of the camera or, if the camera is for underwater photography and is enclosed inside a watertight housing, on the inner or outer surface of such a housing. In this case, a circuit such as an amplifier circuit may be provided in the light receiving part for amplifying the optical signal with reduced intensity received through such a light intensity-reducing filter such that a sufficiently strong electrical signal can be obtained therefrom for activating the outer lamp.
In still another aspect of the invention, use is made of a light guide serving to reflect (and deflect) the light emitted from the light emitting part of the inner lamp and to lead the deflected light therethrough to the light receiving part of the outer lamp while a light-screening means is provided to screen off light propagating forward from the camera. Such a light guide may be pasted onto the light emitting part of the inner lamp. If the camera is for underwater photography and is enclosed inside a housing, the light guide may be pasted on the inner or outer surface of the housing.